In particular, a bypass turbojet for the propulsion of a transport aircraft comprises a fan which is generally upstream and provides an annular flow of air, with a central annular part which supplies the engine with primary flow, and a second, external annular part which is injected into the atmosphere, whilst providing a substantial fraction of the thrust. At present, on the engines, a ring of fixed fins, also known as vanes, is disposed in the secondary flow of air, downstream of the rotor, in order to rectify the flow and increase the thrust provided. These fins are generally designated by the term OGV, which is the acronym for outlet guide vane.
The passage of the air on the structures of the engine causes fluctuating pressure disturbances which are propagated, and form sources of noise which are a nuisance for the environment. In particular, the noise which is propagated in the air inlet duct of a bypass turbojet is caused almost entirely by the fan. In addition, the main source of tonal noise is generally the interaction between the fan and stator elements, whereas the broadband noise is caused in particular by turbulence.
The source of tonal noise is mainly the result of the periodic impact of the wake of the blades of the fan rotor on the fins. It has been proposed, see in particular patent EP1950383, to integrate cavities which form a Helmotz resonator in the stator in order to damp these acoustic excitations. However, this solution makes it necessary to have a certain thickness in the stator. It is not easy to apply, even by using the solution recommended in patent EP1950383, when attempting to obtain fine fins, in order to minimise the disturbances of the flow apart from the rectifier effect.
The noise is amplified if all the parts of the fin are impacted at the same time, and the local contributions are added. In comparison with this, it is known (see for example Journal of Engineering Science and Technology Review 6(1) (2013) 59-61) to provide the stator with forms which are inclined in the axial and/or circumferential direction, in order to assist the dephasing between the sources of noise which are located on the extension of the stator vane, relative to the wake created by the blades of the rotor.
However, these forms are not necessarily optimal from the point of view of the energy performance of the engine. In addition, in the case when the stator is for example a mast or an arm of a casing, these forms pose problems in obtaining structural resistance which makes it possible to withstand the forces sustained by the stator.
A solution considered in the article in the AIAA Journal Vol. 51, No. 11, November 2013 entitled “Experimental and Numerical Investigation of Turbulence-Airfoil Noise Reduction Using Wavy Edges” was to design a leading edge with sinusoidal cut-outs in order to dephase the excitations on the different parts of the leading edge. However, the leading edge is not always the only source of substantial production of noise, in particular when the profile of the stator vane is strongly three-dimensional.